Update devstack install guide for stable branch
Update devstack install guide for stable branch

Existing install guide provides details on liberty
and mitaka is already released now. Instead of changing
liberty to mitaka, guide is updated to be generic enough
to install for the given stable branch.

Change-Id: I74bff43637c0aa3763f703fce070ff76faeffd66

priya-pp/Tacker,zeinsteinz/tacker,trozet/tacker,priya-pp/Tacker,openstack/tacker,stackforge/tacker,openstack/tacker,zeinsteinz/tacker,trozet/tacker,openstack/tacker,stackforge/tacker